Solenoids are widely used throughout the world. Thus solenoids actuate relays or contactors that apply power to the starter motor of most cars. Solenoids actuate the lock mechanism in most keyless door systems. Most automatic valves, whether pneumatic or fluidic, employ solenoids to actuate or pilot the valve. Solenoids are found in factories, buildings, cars and homes.
FIG. 1 depicts a generic solenoid 10 showing its principal constituent parts. The two leadwires, 2, convey electrical current to the solenoid coil 3 which generates a magnetic field. The magnetic circuit of said solenoid 10 includes the metal case 4 and the air gap 6. The armature 5 is influenced by the magnetic field and a force will attempt to move or hold the armature 5 in the direction of the hardstop 8. When said armature 5 contacts and remains in contact with said hardstop 8, it is said to be sealed. Various features are often added to said armature 5 such as the hole 7 in order to attach a mechanism to the armature 5 and thereby complete the mechanical linkage to the solenoid mechanism. Not shown is the return mechanism, such as a spring, which tends to return said solenoid 10 to its open position when electrical current is removed from said solenoid 10.
Solenoids transduce the flow of electrical current into motion via force on the moving portion of the solenoid called the armature. The armature of a solenoid may be connected to various mechanisms, thus in a relay, the armature motion opens or closes electrical contacts whereas in a solenoid-operated valve, the armature is often directly connected to one side of a valve seal. In larger valves, the solenoid operates a smaller so-called pilot valve that employs some fluidic or pneumatic amplification, but the basic operation of the valve is initiated by the solenoid action.
Therefore, solenoids are essential components in a wide range of mechanisms that perform among other things, electrical switching, latching, braking, clamping, valving, diverting or connecting.
The most common method of actuating solenoids involves applying a constant voltage to the coil, whether AC or DC. The voltage causes a current to flow in the coil and a consequent magnetic field is generated which puts force on the solenoid armature and moves the mechanism to which the solenoid is attached. However, as described in detail below, there are significant challenges associated with driving solenoids in an energy efficient manner with circuitry that does not itself create further problems.
FIGS. 2-4 provide examples of circuits used for driving solenoids. FIG. 2 depicts a common prior art transistor solenoid drive circuit including transistor 11 which is capable of conducting electrical current in response to a signal on its input. Said electrical current will flow through solenoid 10. When said transistor 11 is caused to stop conducting in response to a signal on its input, a flyback diode 14 conducts electrical current in order to prevent the inductive component of said solenoid 10 from increasing the voltage seen by said transistor 11 and possibly destroying said transistor 11. When the energy in said solenoid 10 has been exhausted by the recirculation process, said current ceases and said solenoid 10 is thus de-energized.
FIG. 3 depicts a solenoid driver integrated circuit 12 such as is commercially available from a number of manufacturers and employing pulse width modulation (PWM) of the supply voltage in order to reduce the holding current to the solenoid 10. Connected to said solenoid driver 12 is said solenoid 10 as well as two of the commonly required external components, a flyback diode 14 and a series-connected diode 13 intended to both prevent damage to said driver integrated circuit 12 and to somewhat reduce electrical radiation from the PWM switching transients. Said solenoid driver integrated circuit 12 is fixed configuration and cannot be reconfigured for other purposes such as measuring or producing voltages or currents other than required for the narrow solenoid drive task at hand.
FIG. 4 depicts a typical prior art fixed configuration sinking output module 17 capable of driving solenoid 10. As is typical for the prior art, said output module 17 does not provide power to drive said solenoid 10 but instead relies upon connecting and disconnecting power provided by external device power supply 18. In addition, as is customary for said fixed-configuration output modules 17, terminal blocks 19 are employed to effect the wiring to said solenoid 10. In addition, as is customary for said output modules, a protective flyback diode 14 is installed to reduce voltages produced by said solenoid 10 during the de-energization process.
As is widely known to those skilled in the art of solenoid-driven mechanism design, there is a delicate balance between providing sufficient solenoid force at a desired distance of travel and generating excessive energy consumption and heating in the solenoid coil. The amount of electrical current required to move the solenoid to its closed position is high compared to the electrical current required to keep the solenoid closed—or sealed as is the term of art. Thus a solenoid that is to remain sealed for a long period of time tends to become hot and consume a large amount of energy compared to what is needed just to hold the solenoid sealed. The delicate balance for the solenoid-driven mechanism designer is to build a solenoid that will reliably move a given distance to the sealed position while at the same time not consuming excessive electrical power or overheating despite constant application of power to the solenoid coil.
This basic design challenge of the solenoid underscores the problem that is to be solved by this invention, and therefore a more detailed description of the cause of this design challenge is justified in order to explain the merits of this invention.
Whereas the solenoid transduces the flow of electrical current to force on the armature, said force is not a constant function of electrical current. When the solenoid is sealed, there is essentially no air gap in the magnetic circuit, thus the magnetic flux is relatively high at a given electrical current. However, when the solenoid is fully open, there exists an air gap in the magnetic circuit that significantly increases the electrical reluctance of the circuit, said reluctance being the ratio of magnetomotive force (MMF) to magnetic flux developed. Thus at said given electrical current, the force on the fully open armature can be significantly lower than when the armature is in the sealed position. In order to move the armature reliably, therefore, it is necessary to supply more electrical current than is required when the solenoid is sealed. To make matters worse, the requirement for high current to seal the solenoid only lasts for a fraction of a second whereas the solenoid is often left in its conducting, sealed state indefinitely. Energy is being wasted.
Those skilled in the art long ago realized that, for a given solenoid current, the force on the armature increases as the armature moves closer to its sealed or closed position because reluctance decreases with the shorter air gap. These same persons reasoned that by varying the current or voltage to the solenoid, they could provide an initially higher force to seal the solenoid and subsequently reduce the current or voltage in order to hold the solenoid sealed because the force exerted upon a sealed solenoid armature is much higher than the force on an open solenoid given the same electrical current or voltage. By employing this strategy of varying the current or voltage, it is possible to reduce the heating of the solenoid coil while providing the required high force to close the solenoid.
In U.S. Pat. No. 7,262,950 B2 (“Suzuki”), Suzuki teaches that building a current control circuit can allow cutting back the current to the relay coil after the relay has closed. Unfortunately, the circuit of Suzuki requires that a series-wired transistor throttle the current to the relay coil thus creating heat and reducing the possible energy savings considerably. Thus Suzuki's invention does somewhat reduce solenoid heating but by moving some of the heat generation to a transistor. For example, if Suzuki reduced the holding solenoid current to ½ of the initial pull-in current, then the system of Suzuki would see solenoid energy use go down to ¼ of the previous level. Unfortunately, another ¼ of said energy is burned up in ohmic losses in the transistor. In addition, Suzuki does not mention a strategy for dealing with the effect of the relay coil inductance during relay turn-off. It is well understood in the art that employing a transistor to remove power from an inductor will result in a large voltage swing that in general must be mitigated by inserting a path for current to flow thus avoiding a dangerous increase in circuit voltage. Generally, a diode is employed that will allow the relay coil current to circulate during turn-off.
Others have attempted to avoid wasting half of the energy reduction. Others have reasoned that employing pulse width modulation (PWM) of the solenoid voltage could reduce the losses in the transistor via well-understood power switching technology in which the transistor is rapidly turned on and off, largely avoiding its linear region. This strategy works well for inductive circuits wherein little current initially flows during the closing of the transistor. Fortunately, a solenoid is highly inductive, thus PWM works well. Unfortunately, however, PWM can easily generate disruptive electrical radiation unless special care is taken. In an industrial control system application it is almost unthinkable to place restrictions on the user of a solenoid.
Then too, a class of integrated circuits, such as Texas Instruments DRV102 PWM Valve/Solenoid Driver, has aimed to produce a fixed and dedicated electrical circuit capable of initially driving the solenoid with full voltage and consequently full current and subsequently reducing said current by performing PWM of the power signal to the solenoid. Unfortunately, said integrated circuits can produce undesirable electrical interference as described earlier. For example, an application note for the Texas Instruments DRV102 states, “The PWM switching voltages and currents can cause electromagnetic radiation.” The note further suggests that determining the location of noise reducing components “may defy logic”, i.e. may be difficult to predict and require repetitive empirical testing. In addition, such integrated circuits usually require the addition of a number of external components and are fixed configuration: the connector to which the solenoid is attached can only drive a solenoid. The present invention as explained below provides additional applications and flexibility that is not available using these prior art devices.
The prior art has not adequately addressed a significant design challenge in solenoid driving: how to determine if a solenoid is sealed. A solenoid can fail to reach or stay at its closed or sealed position upon the application of electrical current for a number of reasons. The solenoid may be jammed and unable to initially move in either direction. The solenoid coil may be open or not electrically continuous and therefore incapable of generating the required magnetic field. The solenoid coil may be shorted. The solenoid may be exposed to vibration that puts a sufficient force on the solenoid to unseal it. Or, there could be a momentary loss of electrical current that results in the solenoid holding force being reduced briefly. Or, the current applied to the solenoid coil might be slightly less than required to reliably hold the solenoid armature sealed under all physical variations such as ambient temperature. The prior art only teaches a single solution to this dilemma of determining the solenoid state, and that is to cause the solenoid to close an electrical connection when it is sealed. FIG. 5 depicts the prior art apparatus for determining the state of the solenoid, whether sealed or open. In this prior art system, the controller 90 commands a solenoid coil 91 to close. After the solenoid 91 has been given sufficient time to seal, the controller 90 then senses the state of the auxiliary contact 92 which is mechanically linked to the solenoid mechanism. Based upon the state of said auxiliary contact 92, said controller 90 can deduce the state of the solenoid 91. However, if the solenoid 10 is not a relay, then said solenoid 10 must be mechanically connected to said auxiliary contact 92, such connection being problematic and costly. Even in the case where the solenoid is part of a relay, this strategy requires using one set of contacts for this monitoring process. Additional electrical circuits are required to monitor this extra contact, and for systems employing reduced holding current, the actuation sequence must be repeated. In the case where the solenoid is not a part of a relay, then a set of contacts must be added to the solenoid mechanism. This requirement is prohibitive except for the most critical solenoid systems.